Positron annihilation characteristics in UO$_2$: For lattice and vacancy defects induced by electron irradiation

Abstract : Annealing of sintered polished UO$_2$ disks at high temperature (1700DC) has been performed in order to eliminate the defects created both during the sintering and the polishing of the disks. Several sets of such UO$_2$ disks coming from different origins have been studied in the bulk using positron lifetime spectroscopy, and near the surface using a slow positron beam coupled with Doppler broadening spectrometry. They appear homogeneous and similar using positron annihilation spectroscopy. Results show the presence of negative ions, that may be negatively charged oxygen atom located in interstitial site. The positrons annihilation characteristics of the UO$_2$ lattice have been determined and are equal to SL(UO$_2$) = 0.371, WL(UO$_2$)=0.078, $\tau_L$(UO$_2$)=169ps.